A module you are working on needs to send a newsletter to all subscribed customers at predefined intervals in magento 2.
A. Implement MyCompanyMyModuleCronNewsLetterSender::execute and register it in etc/crontab/di.xml
B. Implement MyCompanyMyModuleCronNewsLetterSender::execute and register it in etc/crontab.xml
C. Make sure bin/magento cron:run is added to the system crontab
D. Register the plugin for MagentoCustomerModelCustomer::authenticate in etc/crontab.xml
Explanation: To send newsletter at predefined interval, system cron should be working and this module should have cron registered in magento 2. For more you can check here
You can check my other blog posts for magento 2 here